PUJ stands for "pyelo-ureteric junction", and refers to the point where the renal pelvis of the kidney (the external part of the the kidney's urine collecting system) joins the ureter, the muscular tube which carries urine from the kidney to the bladder. This point can become narrowed or obstructed by scar tissue from a number of causes - congenital (from birth), compression from a blood vessel crossing over this point, scarring from surgery on the kidney (stone surgery usually), or inherent narrowing or fibrosis of no known cause. This problem can be completely silent, and might be detected when a swollen, dilated kidney is seen on a scan done for another reason, or might present with intermittent pain when the patient drinks large volumes of fluid. This occurs because the kidney cannot empty the urine fast enough because of the narrowing, so it swells up and this causes pain.
The diagnosis is made using a combination of the history provided by the patient, scans which examine the anatomy of the kidney (e.g. a CT scan), and scans which examine the emptying ability of the kidney (a renogram). Your urologist will usually order both types of test to determine the diagnosis, any possible causative factors, and to plan surgery.
Treatment of this condition is surgical, with an open or keyhole approach used to remove the narrowed segment and make a new, wider join between the renal pelvis of the kidney and the ureter tube. This is called a "pyeloplasty". A small surgical splint, called a "stent", is left inside the kidney and ureter to allow the join to heal without urine leakage, and is removed a few weeks later with a minor procedure via the bladder. Success is usually around 85%, and most surgeons will order repeat scans over the next 6-24 months to make sure the repair has worked and that no narrowing has recurred.